File:The Quarterly journal of the Geological Society of London (13937153101).jpg
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[edit]DescriptionThe Quarterly journal of the Geological Society of London (13937153101).jpg |
266 MISS C. A. RAISIN ON SOME broken and moved along in a viscous state by a renewed now of the rock, and thus caused to assume an oval form, with more or less lamination of the crust. At Pen-y-chain I found in situ one illustration of spheroidal forma- tion induced around a foreign centre,where the nucleus was a rounded agate-nodule, three inches across, with a thick crust and an interior filled with white quartz. Around this, cutting across the lamination of the perlitic and spherulitic rock, was a crack defining a sphere some 13 inches in diameter. The part within the crack was slightly darkened, but otherwise similar to the mass of the rock. Prof. Bonney describes, in a microscopic slide, a cracking similarly caused by a strain around spherulites in a devitrified glass *. I might mention also, in connexion with these specimens, an example which I found some years ago in a road-heap near Dolgelly. The rock is a kind of diabase, and has a dark ground-mass, in which are imbedded grey, flinty-looking, oval- shaped nodules without radial structure. The matrix is a slaggy-looking mass with deposit of viridite, and contains small felspars of plagioclase form. The nodules are similar, except that viridite is absent from their devit- rified ground- mass, although it occasionally spreads along cracks, or aggregates around what are apparently central cavities filled with quartz. Porphyritic felspars, both orthoclase and plagioclase, occur, generally normal in appearance ; but one crystal seems to be replaced by quartz-grains. There is small epidote, occurring in connexion with dark opaque crystals. Summary. I. At both the headlands of Pen-y-chain and Careg-y-defaid the character of the rocks clearly negatives the theory of intrusion. They are old lava-flows, once glassy, now devitrified, and, at Pen-y- chain, with interbedded agglomeratic and ashy strata. The pro- portion of silica has probably undergone subsequent alteration, but the frequency of porphyritic plagioclase suggests that the rocks approach nearer to dacites than to rhyolites f . As to the slratigraphical position of the Pen-y-chain rocks, apart from any vague suggestion of lithological resemblance, the general dip and the indications of a succession near Llym gwyn seem to afford a tolerable certainty of the Bala age of these volcanic accumulations. The dip is towards the north or east of north, and the rocks therefore belong to the southern part of the synclinal in which the Bala beds of the Lleyn are arranged. As suggested in the Survey memoir J, this synclinal may be traced eastwards, and the ashy beds near Pwllheli may be taken to represent similar strata of Snowdon and Moel Hebog. In like manner we may probably correlate the Pen-y-chain rocks with felsites exposed in those mountains. The mass of felstone also near Pwllheli, examined either in the field or by the microscope, shows Pres. Adtfr. Geol. Soc. 1885, p. 64. t Cf. Kosenbusch, ' Mikr. Pkys. der rnassigen Gesteine,' ii. Abth. p. 418. + Geol. of North Wales, p.218. |
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Source | https://www.flickr.com/photos/biodivlibrary/13937153101 | ||
Author | Geological Society of London | ||
Full title InfoField | The Quarterly journal of the Geological Society of London. | ||
Page ID InfoField | 36940059 | ||
Item ID InfoField | 113696 (Find related Wikimedia Commons images) | ||
Title ID InfoField | 51125 | ||
Page numbers InfoField | Page 264 | ||
BHL Page URL InfoField | https://www.biodiversitylibrary.org/page/36940059 | ||
Page type InfoField | Text | ||
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Flickr tags InfoField |
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Flickr posted date InfoField | 21 April 2014 | ||
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current | 05:47, 26 August 2015 | 1,154 × 2,105 (547 KB) | Fæ (talk | contribs) | == {{int:filedesc}} == {{BHL | title = The Quarterly journal of the Geological Society of London. | source = http://www.flickr.com/photos/biodivlibrary/13937153101 | description = 266 <br> MISS C. A. RAISIN ON SOME <br> broken a... |
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